Add rotate_lines to .config and rotate logs automatically

[psolymos]

Thanks for the great tool. As far as I can see in the source code, the log files are not automatically rotated. I would suggest the following implementation:

• add a rotate_lines=NULL entry to the .config environment
• have functions to set_rotate_lines and get_rotate_lines
• loggit can check if .config$rotate_lines is set, and if it is set to non-NULL, rotate the logs

This would leave the implementation backwards compatible, but would allow using {loggit} in long running sessions where rotate_lines limit is likely to be exceeded, e.g. Plumber APIs etc.

I am happy to work on a PR if this is something you can see as a useful contribution.

[ryapric]

Yep, no auto-rotate is implemented so loggit can be as efficient as possible. #21's implementation is a nice idea though, taking my comments there!